The present invention relates to extensometers used to measure strain in a specimen. More particularly, the present invention provides an extensometer with low contact forces upon the test specimen enabling it to be used on soft or friable materials or tissues.
Measurement of strain in soft materials such as biological tissues or friable materials is a desired yet difficult procedure. Extensometers, which measure the strain in the tissue by measuring deformation, are well known in the art. Standard spring-loaded knife-edge extensometers although quite useful with harder materials such as metal are not suitable since these extensometers may cut the surface of the test specimen thereby influencing the test results. Likewise, strain gages mounted directly to the test specimen are also not suitable even if a glue could be found which did not react with the tissue since mere adhesion of the strain gage to the tissue alters the strain characteristics of the tissue.
Optical based systems wherein contact is not made directly with the test specimen, but rather, by measuring displacement of markers such as dyelines painted onto the surface of the tissue, have been advanced. Although these systems have the advantage of being contactless with the test specimen, the system has some disadvantages. For instance, these systems first require precise positioning of the camera to monitor the markers. Secondly, since the camera sees only a planar view of the test specimen, mere rotation or realignment of the specimen during the test can appear as strain or movement of the markers when actually no strain has occurred.